Langmuir 1996,11, 1353-1360
1353
Structural Characterization of Langmuir-Blodgett Films of a Bridged Polar Stilbene: 2-(4'-( (Perfluoroocty1)sulfony1)phenyl)-6-(Nfl-dimethylamino)benzofuran H. Le Breton, B. Bennetau, and J. Dunoguhs Laboratoire de Chimie Organique et Organomttallique, Universiti Bordeaux I, URA 35 CNRS, 351 Cours de la Libiration, 33405 Talence, France
J.-F. LQtard,R. Lapouyade," L. Vignau, and J.-P. Morand Laboratoire de Photophysique et Photochimie Moliculaire, Universiti Bordeaux I, URA 348 CNRS, 351 Cours de la Libiration, 33405 Talence, France Received October 20, 1994. I n Final Form: December 14, 1994@ The spectral properties of a bridged polar stilbene, the 2-(4'-((perfluorooctyl)sulfonyl)phenyl)-6-(N,Ndimethy1amino)benzofuran(PFSDS-023t)were investigated in mono- and multilayer Langmuir-Blodgett (LB)films. This molecule forms stable films at the air-water interface which can be transferred to give quartz-supported mono- or multilayers. Absorption and emission spectra of the LB film were compared with those in solution. The dependence of the Stokes shift versus the solvent polarity has been measured to evaluate the extent of charge transfer in the excited state. The observed changes in absorption and emission spectra of the LB films can be explained by an exciton model. The tendency of PFSDS-023 to form H-aggregateswas observed even when diluted with C ~ F ~ ~ ( C H ~ ) ~ O(1:18). C H ~The O Hmolecular cross sectional area and the Fourier transform infrared spectroscopy (FTIR)suggest a perpendicular orientation of the dimethylanilino and perfluoroalkyl groups with respect to the substrate.
1. Introduction Among the methods for organizing complex molecules, the Langmuir-Blodgett (LB) technique1 exhibits many potential applications within molecular electronics, nonlinear optics, and conducting thin films. In this context, the structural properties of the LB films may have important consequences for applications. The very high effective concentration of molecules in the LB films leads, in some cases, to aggregation phenomena, which are not detectable in diluted condensed phase^.^,^ As a rule, different types of aggregates can be produced, ranging from these whose transition moments are aligned a t a n angle close to 90" to their line of center (H-aggregates, blue-shifted absorption, red-shifted emission) t o those having an angle lower than 30" (J-aggregates, red-shifted absorption and e m i ~ s i o n ) . ~ In previous studies, Whitten et aL5a6have found that surfactant derivatives containing the trans-stilbene chromophore as a component of the hydrocarbon backbone of fatty acid molecules (nonpolar stilbenes) form H-aggregates exhibiting blue shift in their prominent absorption transition and red-shifted emission spectra. On the contrary, 4-(octadecylamino)-4'-nitrostilbene(a polar stilbene) was reported to form J-aggregates with a bathochromically shifted absorption and e m i s ~ i o n .Recently, ~ + The numbers 2,3 indicate the position of the five-membered ring relative to the NMez group, as in the nomenclature proposed in ref 24 and presented in Scheme 1. The symbol " 0represents the oxygen heteroatom incorporated in the five-membered ring,
which bridges the central double bond ofthe stilbene chromophore. Abstract published in Advance ACS Abstracts, February 15, 1995. (1) Roberts, G. G. Contemp. Phys. 1984,2,109. (2) Barber, D. C.; Freitag-Beeston, R. A.; Whitten, D. G. J. Phys. Chem. 1991,95,4074. (3) Whitten, D. G. Ace. Chem. Res. 1993,26,502. (4)Kasha, M. Radiat. Res. 1963,20,55. (5) Mooney, W. F., 111; Brown, P. E.;Russel, J. C.; Costa, S. B.; Pedersen, L. G.; Whitten, D. G. J. Am. Chem. Soc. 1984,106,5659. (6) Moonev. W. F.. 111: Whitten, D. G. J.Am. Chem. SOC.1986,108, 5712.
*
0743-746319512411-1353$09.0010
Whitten et aL8have observed for a series of polar stilbenes containing in para and para' positions (o-hydroxycarbony1)hexadecanyl and alkylsulfonyl substituents, respectively, the formation of H-aggregates with hypsochromic absorption shift, but no emission shift of the LB monolayer relative to that of in chloroform solution. One of their interpretations is that the increasing of the excited state dipole moment, upon excitation, induces strong repulsive interactions between cofaciallypacked adjacent excited- and ground-state molecules.* In the present paper, we report the synthesis and the structural characterization of the mono- and multilayers LB films of a bridged polar stilbene, 2-(4'-((perfluorooctyl)sulfonyl)phenyl)-6-(N~-dimethylamino~benzofuran (PFSDS-023, Figure 1). This stilbene bears donor (-Me21 and acceptor (-SOZ(CFZ)&F~) substituents, which are very strong. It was recognized that donor-acceptor (D-A) substituted stilbenes possess favorable properties in the field of second-order nonlinear optics: and while nitro and polycyanovinyl groups have been widely used as acceptors, the sulfonyl group has not received large attention despite (i)its strong acceptor properties,1° since its opand 0- are +0.72 and +1.05, respectively, while for the nitro group these values are +0.79 and +1.24, respectively, and (ii) its large in-plane dipole moment,ll which leads to a considerable stabilization of the assembly in the LB films from the Coulombic interactions. We chose a perfluoroalkylsulfonyl group because (i)recent studies have shown that such a group affords favorable trade-off between quadratic optical nonlinearity and optical transparency suggesting promising uses in the frequency conversion of (7) Lippitsch, M. E.; Draxler, S.; Koller, E. Thin Solid Films 1992,
217. 161.
(8fFurman,I.; Geiger, H.C.; Whitten, D. G.; Penner, T. L.; Ulman, A. Langmuir 1994,10,837. (9) Oudar, J. L. J. Chem. Phys. 1977,67,446. (10)Kosower, E.M. Physical Organic Chemistry; Wiley: New York, 1968. (11) Cheng, L.-T.; Tam, W.; Feiring,A.; Rikken, G. L. J. A. Nonlinear Opt. Prop. Org. Mater. 1990,3, 203.
0 1995 American Chemical Society
Le Breton et al.
1354 Langmuir, Vol. 11, No. 4, 1995
PFSDS-023
DS
DCS
Figure 1. Molecular structure of D-A stilbenes and bridged model compounds, in which the central double bond is incorporated in a five-membered ring containing oxygen heteroatom.
short wavelength diode lasers" and (ii) LB filmscontaining fluorocarbon chains have been described to possess characteristic properties such as low friction, excellent insulation, and excellent durability.12J3 PFSDS-023 is a new polyphilic compound with a dimethylamino hydrophilic head, a stilbene rigid core, and a hydrophobic perfluorinated chain (Figure 1). In this molecule, the central double bond is incorporated in a five-membered ring, in order to prevent the trans-cis photois~merization~~ and, thus, to increase the photostability of the product.
Scheme 1 9H
-1 1-NaH
eon
MF. POCC
2. Experimental Section Materials. The course of the reactions and the punty of the final products were monitored by thin-layer chromatography (TLC). Melting points were determined with a Mettler FP62 apparatus. lH NMR spectra were recorded on a Perkin-Elmer R24B (60 MHz) and on a Bruker AC 250 (250 MHz) instrument in CDC13 (Aldrich) solution (shift 6 ppm, tetramethylsilane as the internal standard; multiplicity: s, singlet; d, doublet; dd, doublet of doublet; t, triplet; m, multiplet). 13C NMR spectra were recorded on a Bruker AC 250 (shift 6 ppm; multiplicity: C,, quaternary carbon). Mass spectra were recorded on a VG Micromass 16F (70 eV) apparatus and the most important peaks are given with their relative intensity. C ~ F I ~ ( C H ~ ) ~ O Cwas H ~prepared OH by reduction of the corresponding acid, which was a generous gift from Professor A. Commeyras and Dr. H. Blancou, University of Montpellier 11. The surfactant stilbene PFSDS-023 was synthesized by the route outlined in Scheme 1. The last step involves a Knoevenagel reaction between 4-(dimethylamino)-2-hydroxybenzaldehyde (6) and p-((heptadecafluoroocty1)sulfonyl)benzyl bromide (4). p-((Heptadecafluoroocty1)thio)toluene (2). A solution of p thiocresol (1)(5 g, 40 mmol) and NaH (0.96 g, 40 mmol) in dry DMF (30 mL) was stirred under nitrogen. Heptadecafluorooctyl iodide (21.9 g, 40 mmol) was rapidly added. After being stirred for 3 h at room temperature, the mixture was treated with HC1 5 M (60 mL), extracted with ether (3 x 30 mL), and then washed withNazSz03 0.6 M (10 mL). The ether extract was dried (MgS04) and concentrated to give a colorless liquid (20.7 g, 38.2 mmol, 95% yield). bp0.05 80 "C; lH NMR (60 MHz) 6 2.3 (s,3H, -CH3), 7; 7.1; 7.4; 7.5 (4H,, syst. AAXX'); MS ( m l e ) 541.9 (86), M + (molecular peak); 173 (331, f!7F15; 123 (loo), *C8F17; 91 (411, 'SC8F17. p-((Heptadecafluoroocty1)sulfonyl)toluene(3). Perfluoroalkyl thioether (2) (5 g, 9.2 mmol) was added to a stirred solution of m n 0 4 (4.4 g, 27.6 mmol), HzO (2 mL), triethylamine (0.93 g, 9.2 mmol), and CHC13 (40 mL) cooled in an ice bath. Then HzS04 6 M (12 mL) was added. After 2 h at a temperature lower than 10 "C, the mixture was passed through a silica gel (10 g ) and CaCl2 (2 g) with CHC13 as eluant. The solvent was evaporated and the isolated product was recrystallized (cyclohexane)to give a white powder (3.7 g, 6.4 mmol, 70% yield): mp 79 "C; lH NMR (60 MHz) 6 2.4 (s, 3H, -CH3), 7.3; 7.85; 8.05; 8.2 (4H,, syst. AAW). p-((Heptadecafluoroocty1)sulfonyl)benzyl Bromide (4). A solution of (3)(3 g , 5.22 mmol) in CC14 (10 mL) was stirred under reflux. Then NBS (0.93 g, 5.22 mmol) and dibenzoyl peroxide (0.12 mg, 0.5 mmol) were added. The solution was stirred and refluxed for 6 h. Then the mixture was cooled at room ~~
(121"redgold, R. H.; Smith, G . W. Thin Solid Films 1983,99,215. (13) Clint, J. H.; Walker, T. J. Colloid Sci. 1974,47, 172. (14)Orlandi, G.; Siebrand, W. Chen. Phys. Lett. 1976,30,352.
-3
-4I
PFSDS-023 temperature and filtered off. The filtrate was evaporated to dryness and the final product was purified by chromatography on silica gel (3/1 pentane/CHCla (v/v))to give a powder of (4) (1.4 g, 2.15 mmol, 41% yield): mp 72 "C; lH NMR (60 MHz) 6 4.5 (s, 2H, -CHZBr), ), 7.7; 7.85; 8.05;8.2 (4H,, syst. AAXX');MS (mle) 652.9 (691, M'+ (molecular peak); 234.9 (191, 'C8F17; 169.9 (611, *S02C8F17;89 (1001, HBr; 69 (46). 2-Hydroxy-4-(N,N-dimthylamino)benzaldehyde (6):Poc13(5.6 g, 36.4 mmol) was added dropwise to DMF (20 mL) cooled in a n ice bath. The solution was stirred for 1h a t 0 "C and 1h at room temperature. Then (5) (5 g, 36.4 mmol) was slowly added and the mixture was heated at 90 "C for 6 h. The cooled solution was poured onto 50 g of ice, extracted with CHZC12, washed with a saturated solution ofNaHCO3, dried over MgS04, and evaporated to dryness to give (6)(5.4 g, 32.8 mmol, 90% yield). lH NMR (60 MHz) 6 2.9 (8,6H, -N(CH&), 6.1 (s,1H, Ha), 6.3 (d, 1H, Hb, Jbe = 9 Hz), 7.3 (d, l H , &, Jcb = 9 Hz), 9.6 (s, l H , -CHO), 11.7 (s, l H , -OH).
Langmuir, Vol. 11, No. 4, 1995 1355
Bridged Polar Stilbene Films PFSDS-023. A solution of (4) (0.33 g, 2 mmol),(6)(1.5 g, 2.3 mmol), dry &co3 (1.8 g), and Bu4NHS04 (0.16 g) in DMF (25 mL)was stirred and heated at 125 "C in aoil bath, under nitrogen atm, for 4 h. Then the mixture was poured onto water (50 mL) and the product was extracted with CHzClz (2 x 30 mL), dried over MgS04, and the solvent evaporated. The crude product was purified by chromatography on silica gel (CH2C12) and ether (v/v) to give a recrystallized from 1/1 CH~C12/petroleum yellow powder (1 g, 1.4 mmol, 70% yield): mp 160 "C; lH NMR (250 MHz) 6 3 (s, 6H, N(CH3)2), 6.7-8.0 (m, 8H, C,-H); 13C NMR (250 MHz)6 41 (N(CH&, 94, 106.7, 110.9, 121.8, 124.3, 131.7 (C-H), 118.9,129.3,139,150.7,150.8,158.3 (C,J; IR(KBr) v (cm-l) 1629, 1593, 1370, 1216, 1151. Apparatus and Methods. Absorptionand emissionspectra were recorded on a Hitachi U-3300 and Hitachi F-4500, respectively. Fluorescence spectra of the LB films were observed by orienting the slides in a custom-builtholder, at a 45" angle with respect to the excitation source, and the detection was effected at right angles. Solution-phase fluorescence spectra were obtained in 1-cm-path-lengthquartz cuvettes with right angle detection. Fluorescence lifetimes of PFSDS-023 solutions and mono- and multilayers were determined using time-correlated single photon counting described elsewhere.16 For Fourier transforminfrared (FTIR)measurements with a Nicolet 20 SXC, films were deposited onto 30 x 10 x 2 mm3 optically polished calcium fluoride plates. The FTIR experimentswere carried out at normal incidenceof the substrate. The light wave vector was parallel to OZ, while the electricvector E was polarized parallel to the plane OXY of the substrate. Monolayer Film and MultilayerDeposition Techniques.
The general methods used for the preparation of LangmuirBlodgett films and assemblies as well as the cleaning of the substrates were based on techniques described elsewhere.16We used ultrapure water purified in a Milli-Q-System(Millipore)as subphase (resistivity 18 MR-cm). Stock solution concentration of PFSDS-023 surfactant was 5 x mol-L-l in solution of chloroform (SDS, spectroscopy grade). This solution (0.25 mL) was spread on the subphase surfaceand the chloroformremoved by evaporation over 30 min prior to compression. Then the monolayer was compressed at room temperature at a barrier speed of 2.5 mdmin. The transfer onto quartz slides (20 x 7 x 1mm3)was achieved at 25 mN/m for a correspondingmolecular area close to 38 A2/molecule,using an extremely low transfer speed (1.75 mdmin). 3. Results and Discussion
Compression Isotherm. Figure 2 shows the surface pressure-area isotherm of PFSDS-023. The steeply inclining part corresponding to the formation of the solid monolayer and the high surface pressure of the collapse point of the monolayer (around 40 mN/m) indicate the good film-forming behavior of the (perfluoroalkylsulfony1)stilbene molecules, as described recently for F3C(CF~)~(CHZ)~OCH~OH, which forms a monolayer at the airwater interface with a collapse pressure around 41 mN/ m.17 The linear solid phase portion of the isotherm extrapolated to zero surface pressure gives the area per molecule, i.e. about 42 A2/molecule. This value appears to be larger than the 33.2 A2/molecule for perfluoroundecanoic acid18 (CloFzlCOOH) or the area per molecule for surfactant derivatives containing the trans-stilbene chromophore, 22 ~z/moleculelg or the estimated value for molecular cross(15) Brotin, T.; Desvergne, J.-P.; Fag&, F.; Utermohlem, R.; Bonneau, R.; Bouas-Laurent, H. Photochem. Photobiol. 1992,55, 349. (16)Kuhn, H.; Mobius, D.; Bucher, H. In Physical Methods of Chemistry; Weissberger, A., Rossiter, B. W., Eds.; Wiley: New York, 1972; Vol. 1, p 577. (17) Dupart, E.; Agncole, B.; Ravaine, S.;Mingotaud, C.; Fichet, 0.; Delhaes, P.; Ohnuki, H.; Munger, G.; Leblanc, R. M. Thin Solid Films 1994, 243, 575. (18)Nakahama, H.; Miyata, S.; Wang, T. T.; Tasaka, S. Thin Solid Films 1986, 141, 165. (19) Spooner, S. P.; Whitten, D. G. J. Am. Chem. SOC.1994, 116, 1240.
Pressure [ mNIm
T
20
40
Ares
60
[i2/ Molecule J
Figure 2. Compression isotherm of films of PFSDS-023 at
the air-water interface at room temperature. sectional area (28 A2) of the fluorocarbon chain.20 A possible explanation for this increased area can be suggested: (i) Fluoroalkyl chains, such as -(CF2),CF3, can form a helical structure, as observed for polytetrafluoroethylene I1 crystals which form 15/7 helixes21 and then a slight increasing of the molecular cross-sectional area can be expected. (ii)Another interpretation is that PFSDS-023 molecules are in a tilted arrangement. An atomic model of PFSDS-023, obtained with the molecular mechanics program PC Model 4D (Serena Software, Bloomington, IN), is depicted in Figure 3a. After a MMX minimization, based on the MM2 force field of Allinger (QCPE-395; 19771, an angle of about 36" was found between the axis ofthe perfluoroalkylsulfonylphenyl group and the C-N bond, as shown in Figure 3b. This value is close to 40") which is the angle expected from comparing to the X-ray structure of a benzofuran derivative substituted in 2,6 positions.22 Assuming that the dimethylanilino group is perpendicular to the quartz surface, the perfluoroalkyl chain direction formed with the last fourth CF2 and the CF3 groups is also at a right angle with the interface (Figure 3a). The molecular cross-sectional area of PFSDS-023 in this perpendicular orientation can be estimated from the largest cross section of PFSDS-023 calculated with the highest width of the molecule (8.9 distance between the two opposite hydrogens of the phenyl rings taking into account their van der Waals radius, as presented in Figure 3b) and the thickness of PFSDS-023 (6 A for the perfluoroalkyl group20). Then the calculated area of this ellipse is [(8.9/2)(6/2)n]= 41.9 Az. This value is close to the experimental one (42 A2/molecules) and therefore supports the hypothesis of a perpendicular arrangement of the molecules relative to the quartz support. Absorption Spectra. The absorption spectrum of the monolayer of PFSDS-023, transferred to a quartz support, is shown in Figure 4 and compared to a spectrum of a solution in n-hexane.
A,
(20) Laschewsky, A.; Ringsdorf, H.; Schmidt, G. Thin Solid Films 1986,134,153. (21) Wunderlich, B. Macromolecular Physics;Academic Press: New York, 1973; Vol. 1,p 97. (22) Chaichit, N.; Gatchouse, B. M. Cryst. Struct. Commun. 1981, 10, 83.
1356 Langmuir, Vol. 11,No.4,1995
Le Breton et al. I (u.a.)
a)
400
300
7
500
600
Wavelength (nm) Figure 4. Normalized absorbance and fluorescence spectra of PFSDS-023 in n-hexane (-) and in LB monolayer on quartz
substrate (-
-).
specific interactions in a ( A ~dichloromethane = 990 cm-l).
chlorinated
solvent
a n-hexane
Figure 3. (a)Atomic model for one rotamer of the PFSDS-023 molecule obtained with the molecular mechanics program PC Model 4D. (b)A schematicdiagramof one molecule. The dashed lines show the normal to the substrate and the orientation of the perfluoroalkylsulfonylphenylgroup on the normal to the
substrate.
In an organic solvent, the absorption spectra of PFSDS0 2 3 (Figure 5 ) is dominated by a broad, intense band centered at 407 nm (n-hexane), which resembles that of stilbene with a considerable bathochromic shift. The absorption maxima of PFSDS-023 in various solvents are reported in Table 1. The position of this maximum of absorption is slightly affected by the solvent polarity; for instance from nonpolar solvent (A!$:[n-hexane] = 407 nm) to polar solvent (I!$:[acetonitrile] = 418 nm), we observe a bathochromic shift of AFa = 650 cm-l (0.08 eV). This small variation of A!$,, when the polarity of the solvent increases, indicates a small difference between the dipole moments of the Franck-Condon excited and ground states, as observed for others D-A Nevertheless, PFSDS-023 undergoes strong
The absorbance spectrum of the monolayer (A!$: = 356 nm), transferred a t 25 mN/m, is blue-shifted relative to that in n-hexane (2%: = 407 nm). If we transfer at a lower pressure (8mN/m) or at a pressure higher than 25 mN/m, i.e. 35 mN/m, the spectra of the LB films are not altered, showing that the arrangement observed in the LB film must represent a minimum free energy conformation for the system. This blue shift (4500 cm-l) is consistent with the formation of H-aggregates, characterized by a parallel arrangement of the stilbene chromophores in a direction perpendicular to the ~ u b s t r a t e .This ~ interpretation was based on the Kasha model for dipole-induced splitting of excited-state energy levels, where the absorption corresponds to the higher energy-allowed state t r a n ~ i t i o n . ~ LB films formed by a 1:4 and 1:18 mixture of PFSDS0 2 3 and C8F1,(CH2)1&H20H display the same absorbance spectrum, indicating the strength of H-aggregates formation. A similar aggregation behavior, as indicated by absorption, is observed for supported mono- and multilayers or several other stilbene s~rfactants~9~9~ (where the perfluoroalkyl chain is absent) and shows that these D-A stilbenes are capable of self-association, even in diluted conditions, probably driven by apolar association forces between the long alkane (or perfluoroalkane) chains. Figure 6 depicts the absorbance at 350 nm versus the number of layers of LB films. The relationship between the absorbance and number of layers, which is linear up to 14 layers, demonstrates a constant transfer ratio during the sequential deposition and a constant architecture in the multilayer LB films. Emission Properties. Figure 4 shows the emission spectra of PFSDS-023 in LB films and in n-hexane solution. The monolayer fluorescence spectrum exhibits a strong blue-shift relative to that in n-hexane. In solution, the position of the emission maximum is very sensitive to the polarity of the solvent and the results are collected in Table 1. Figure 5 shows normalized fluorescence spectra of PFSDS-023, measured in three solvents of different polarity. In polar solvents, the spectra are completely structureless. The red-shift of the fluorescence maxima (23) Lapouyade, R.; Czeschka, W.; Majenz, W.; Rettig, W.; Gilabert, E.; Rullibre, C. J . Phys. Chem. 1992, 96, 9645. (24) LBtard, J.-F.;Lapouyade, R.; Rettig, W. J.Am. Chem.SOC.1993,
115,2441.
Bridged Polar Stilbene Films
Langmuir, Vol. 1 2 , No. 4,1995 1357
- - - n-hexane - - diethylether
Intensity 1a.u.)
*
350
300
400
450
500
550
600
650
700
750
Wavelength (nm) Figure 5. Normalized absorbance and emission spectra of PFSDS-023in three solvents at room temperature: n-hexane (-
diethyl ether (- -), and acetonitrile (-). Table 1. Absorption (,I$:, nm) and Fluorescence (,IF:, nm)Maxima and Difference between Absorption and Fluorescence Maxima (AVst, cm-‘) for PFSDS-023 at Room Temperature in Solvents of Different Polarity (An and in the LB Films solvents
n-hexane Bu~O CHC13
Et20 THF CHzClz
DMF
CH3CN LB films
Amax
flu0 Amax
Acst
407 419 423 418 426 424 430 418 356
449 508 536 529 574 571 620 611 440
2298 4181 4984 5020 6053 6072 7127 7557 5363
ab8
4@ 0 0.096 0.148 0.167 0.210 0.218 0.275 0.305
The parameter of polarity, used in the Lippert equation,25is defined according to Af= [(c - l)/(2t 1)1 - [(n2 - 1)/(2n2 111).
+
+
- -1,
Table 2. Solvatochromic Slopes nl (cm-VLV) of D-A Stilbenes and Coefficient of Autocorrelation ( r )Obtained According to Equation 1 Taking into Account the Solvents of Table 1, Except for DS and DCS Where the Chloroform Was Missed, the Onsager Parameter “d’of the Stilbene Derivatives Calculated from the Volume of the Solute, Which was Considered as a Rectangular Parallelepiped, and the Excited State Dipole Moment Ole, Debye Units) Obtained According to Equation 1, with the Correspondng Ground State Dipole Moment Ole Debye Units)
compounds
ml
r
PFSDS-023 DCS DS
17 134 17 356 11397
0.998 0.988 0.989
a& 5.6 4.4a 4.2a
Pe
5.2 6.45b 2.4lC
22.2 18.4 11.4
a For estimating the Onsager parameter, the lengths of DCS and DS used are, respectively, 17.4 and 14.8 A. “he width and the thickness are 6.2 and 3.4 A. Reference 29. Reference 30.
stilbene, where the influence of the long perfluoroalkyl chain, which is not part of the auxochromic group, and the effect of the oxygen atom in the five-membered ring are neglected. The Onsager radius “a”for this trifluoromethylsulfonyl derivative is estimated to be [(3 x 18.6 x 6.7 x 6)/(4~)1l’~ = 5.6 i.e. from the length ofthe molecule (18.6 A from the hydrogens of the dimethylamino substituent to the fluorine atoms of the CF3 group, taking into account their van der Waals radius), the width (6.7 A, distance between the opposite hydrogen of a phenyl ring), and the thickness (6Afor the perfluoroalkyl group20). The value of the ground-state dipole moment can be calculated using a vectorial method from Nfl-dimethylaniline kg = 1.3DZ7) and the ((heptafluoropropy1)sulfonyl)benzene with an angle of 36” (Figure 3b). The groundstate dipole moment of the ((heptafluoropropy1)sulfonyl)benzene (pg = 4.1 D) was derived from the pg of the pmethoxy((heptafluoropropy1)sulfonyl)benzene(pg= 5.4 Dl1),with 1.3D for the methoxy substituent of an aromatic compound.28 The ground-state dipole moment of PFSDS0 2 3 is then 5.2 D, assuming identical effect between -C8F1, and -C3F7 and neglecting the effect of the oxygen atom in the five-membered ring. According to eq 1, a plot of AVSt(in cm-l)versus Af should be linear with a slope ml = 2Api.Jhca3. Table 2 collects
A,
“0
2
4
6
8
10 12
14
Number o f layers
Figure 6. Absorbance at 350 nm as a function of the number of LB layers of PFSDS-023.
versus the solvent polarity can be assigned to a strong increase of the charge transfer in the excited state. The dipole moment ofthe excited state Ole)is determined by the solvatochromic method, using the Lippert equationZ5
where Apegis the difference between excited and ground state dipole moments and Af = [ ( E - 1)/(26 l)]- [(n21)/(2n2 l)]. This equation is based on the Onsager description of a solute-induced reaction field inside a spherical solvent cavity of radius “a”.z6For this parameter, we took the value obtained for the model compound, 4 4 (trifluoromethyl)sulfonyl)-4’-(N,N-dimethylamino)-
+
+
(25) Lippert, E. 2.Nuturforsch. 1958, loa, 541. (26) Onsager, L. J.A m . Chem. SOC.1938,58,1486.
(27) Nagakura, S.; Baba, H. J . Am. Chem. SOC.1952, 74, 5693. (28) Smith, J. W. Electric Dipole Moments; Buttersworths Scientific Publications: London, 1955; p 96.
1358 Langmuir, Vol. 11, No. 4, 1995
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Le Breton et al.
c
1
4
LW Cwntr
0
C t u m a I nurbmr
DE"
Figure 7. Fluorescencedecay (Aexc = 370 nm) of the emissionof a PFSDS-023monolayer at 440 nm. The experimentalfluorescence decay and the instrument response function are given by the point plot. The calculated decay (-) was fitted by a monoexponential decay (channel width 0.025 ns) with x2 and Durbin-Watson parameters listed in Table 3. Table 3. Fluorescence Decays (y f 0.1 ns) of the Emission of PFSDS-023 in Solution and in LB Films at Room Temperature, Statistic Test 2,and Durbin-Watson Parameter (DW) solvents &xc lobs Zf x2 DW
n-hexane diethyl ether LB films
370 370 370
450 500 440
1.9 2.8 1.3
1.19 1.13 1.18
1.81 1.70 1.82
the slopes ml (Aii,JAf, of eq 1, the Onsager parameters, and the excited and ground state dipole moments for PFSDS-023 and some related D-A stilbenes. From the measured slopes and the estimate of "a",values ofp, may be obtained. The deduced excited state dipole moment of PFSDS-023 = 22.2 D) is only approximate owing to (i) the difficulty to estimate the Onsager parameter and (ii) the direction of the dipole moment in the excited state is certainly different from that in the ground state. Nevertheless, the dipole moment of PFSDS-023 in the excited state (22.2 D) is larger than those obtained with 4-(NJV-dimethylamino)stilbeneb,(DS) = 11.4 D) and 4-cyano-4'-(N,N-dimethylamino)stilbene Ge(DCS)= 18.4 D), as expected, due to the strong donor-acceptor intera~tion.~~,~~ The emission spectra of the monolayer :;.I( = 440 nm) show a hypsochromic shift (460 cm-') relative to that in n-hexane (I.;: = 449 nm) solution (Figure 4). While H-aggregates should lead to a red shift emission,4Whitten8 had observed a similar emission for D-A stilbenes (D = -O(CH21nC02H and A = - S O Z C ~ H ~ ~in+chloroform I) and in monolayer. One of his explanations was a destablization of the excited state from repulsive Coulombic interactions with neighboring ground-state molecules. The blue shift that we observed with PFSDS-023 is larger than the one observed by Whittens and could result from higher dipole moments of the ground and excited states. Fluorescence Decay. Figure 7 shows the fluorescence decay curve of LB monolayer obtained by monitoring a t 440 nm. The fluorescence decay curve is single-expo-
cue
( 2 9 ) Kawski, A. 2.Nuturforsch. 1977, 32u,420. (30)Everard, K. B.;Kumar, L.; Sutton, L. E. J. Chem. SOC.1961, 2807.
50 WFIVENUMRER
Figure 8. Infrared spectra of PFSDS-023 in multilayers (- - -1 and in bulk KBr (-1.
nential with a lifetime of 1.3 ns, whereas in solution the lifetimes are slightly larger (Table 3). Fluorescence lifetimes of several nonpolar trans-stilbene multilayers were measured by Whitten et al.;5in every case the major fluorescence lifetime component was found to be in the order of 3-5 ns, which represents approximately a 50fold increase over the fluorescence lifetimes of transstilbene in solution (zf = 60 ps3). This increase for unbridged nonpolar stilbenes can be explained by the suppression of the trans-cis photoisomerization in LB films5 and by the decrease of the radiative constant (forbidden transition4) in H-aggregate. Infrared Characterization. Figure 8 displays the absorption spectra found for PFSDS-023 in multilayers and dispersed in KBr. Table 4 collects the different frequencies and relative intensities normalized a t vas(CF2) band. In the high-frequency region (3000- 1900 cm-l) the CH stretching absorptions of aromatic groups occur a t 3111-
Bridged Polar Stilbene Films
Langmuir, Vol. 11, No. 4,1995 1359
Table 4. Frequencies of the Infrared Spectra of PFSDS-023in LB Films and in Bulk KBr, Intensities Relative to the v,(CF2) Ratio of Intensity Normalized at the C-F Stretch in (CF2). Degenerate Mode between the Bulk KBr and the LB Spectra (R = ZL$lgBr) and Assignments of the Different Frequencies wavenumber in cm-' wavenumber in cm-I (relative intensity) (relative intensity) ratio of of bulk KBr spectra of LB spectra intensity IL$ID? assignment 3111 (1) 3103 (1) 1 }C-H stretching of aromatic groups 2957 (0.19) 2925 (0.23) 2092 (0.13) 1937 1629 (0.58) 1593 (1.03) 1510 (0.29) 1484 1445 (0.13) 1407 (0.19) 1371 (0.94) 1327 (0.13) 1290 (0.19) 1248 (0.55) 1216 (1) 1150 (0.97) 1118 (0.39) 1082 (0.29)
2961 (0.09) 2933 (0.07)
0.47 0.30
1627 (0.16) 1594 (0.32) 1511 (0.10)
0.28 0.31 0.34
ivdCH3) and vs(CH3) overtone of &CH) (out of plane) and combination C-C stretching in phenyl groups presumably C-C stretching in phenyl groups
1405 (0.07) 1368 (0.40)
0.37 0.43
1209 (1) 1151 (0.93) 1120 (0.28) 1083 (0.16)
1 0.96 0.72 0.55
3088 cm-l for FTIR LB films and KBr spectra, with a very low intensity. The relative intensities of the va,(CH3) and the v,(CH3) stretchings oftheNMez group, for KBr spectra, in the range of 2957-2857 cm-l, are strongly decreased (R= 0.46) in the LB films. The mid-frequency region (1700- 1000 cm-') contains most of the information regarding the phenyl vibrations and those of the donor and acceptor substituents. Figure 8 shows four vibrations for the KBr spectra a t 1627 cm-' (medium intensity), 1594cm-l (strong), 1511cm-l (weak), and 1484 cm-l (weak), which can be assigned to phenyl vibrations. Normally, typical vibrations of phenyl rings are a group of four bands between 1650 and 1450 cm-', weak vibrations at 1600 cm-l (CZ", all and 1575 cm-l (CzU, bz), a strong vibration a t 1500 cm-l ( C Z ~all, , and a weak vibration close to 1475 cm-' (CZ,, bz), where the a1 species of vibrations are parallel to the main axis of the phenyl group, while the bz species are perpendicular to it. However, a recent study of Ulman et al.31about phenyl sulfone and sulfide derivatives shows that the -SOzgroup induces a n inversion in the ratio of the intensity, assigned to the strong electron-withdrawing properties, which when coupled with the electron donor leads to an enhanced conjugation of the ring. The shift of 27 cm-l to higher energies for the two high-frequency phenyl vibrations (1627 and 1594 cm-') is in good agreement with a para-substitution ofthe phenyl The most notable change in this region between the multilayers films and the bulk spectra is the decrease of the relative intensities (R= 0.35-0.28) for the C-C vibration and the absence of the fourth skeletal C-C frequency in the region of 1480 cm-l. Such a behavior can be interpreted qualitatively by a more highly ordered structure where the transition moment of phenyl vibrations, oriented parallel to the main axis of the phenyl group (vibrations, Czu,a d , lies more parallel to the normal of the surface. The asymmetric sulfone stretching vas(SOz) appears strong around 1371 cm-l and its symmetric counterpart vs(S02)at 1118 cm-l. In fact, this assignment is in the range of the sulfone vibrations (vas(SOz) 1350-1300 cm-I and v,(SOz) 1160-1120 cm-l), obtained by S~hriebel.3~ and (31) Evans, S. D.; Urankar, E.;Ulman, A,; Ferris, N. J.A m . Chem. SOC.1991,113, 4121. ( 3 2 )Colthup, N. B. J. Opt. SOC.Am. 1950,40,397. (33) Schrieber, K. C. Anal. Chem. 1949,21,1168.
VEdSOZ) vas(C-F) stretching in CF3 Vas (C-F) stretching in (CF2) vs (C-F) stretching in (CFd va
(sod
presumably in-plane aromatic C-H
Barnard et al.,34who have compared the spectra of seven alkyl sulfones a t the solid state and in solution. The shift of 20 cm-l toward higher frequencies for the vas(SOz)is in good agreement with observations from aromatic sulfones (between 1376 and 1358 ~ m - l ) .The ~ ~ infrared spectra of the multilayer films show a strong decrease of the relative intensity for the vas(SOz)(R = 0.42) and a lower one for the vs(S0z)(R= 0.72). This result indicates a structure where the transition moment of the Y,,(SOZ) is oriented closer to the 2 axis than for its counterpart v,(SOz). The 1248-cm-l band is assigned to the CF3 group. The asymmetric and symmetric CFZstretching vibrations of PFSDS-023 appear a t 1216 and 1150 cm-l, respectively. The intensities of these two bands, characteristically strong5in FTIR KBr spectra, are unchanged in FTIR LB spectra. Such behavior provides strong evidence that the perfluoroalkyl chain is perpendicular to the surface of the calcium fluoride substrate, as proposed from comparison of the calculated and measured molecular area and analyzed in the next section. Orientational Analysis. Given a rectangular coordinated system (X,Y,Z) associated with the calcium fluoride substrate with 2 along its normal, one has to find the orientation of the transition dipole moments of the observed bands. In the case oftransmittance experiments under normal incidence, the electric field is oriented in the OXY plane. Consequently, each vibrational mode (i) has the intensity proportional to the projection of Mi2 in the plane OXY.36 If we define pi, the angle between the transition moment of a particular vibration mode i and the 2 axis, it can be written:37
where Zi is the relative intensity given by the FTIR LB spectra and Ai is the integrated IR absorbances that can usually be measured by transmission through a bulk (34)Barnard, D.; Fabian, J. M.; Koch, H. P. J. Chem. SOC.1949, 2442. (35)Bellamy, L.J. The Infrared Spectra Complex Molecules, 3rd ed.; Chapman and Hall London;1975; Vol. 1, p 405. (36)Greenler, R. G. J.Chem. Phys. 1966,44,310.
(37) Colthup, N. B.;Daly, L.H.;Wiberly, S. E. Introduction to Infrared and Raman Spectroscopy, 3rd ed.; Academics Press: New York, 1990; p 41.
Le Breton et al.
1360 Langmuir, Vol. 11, No. 4, 1995 sample. Then, the intensity ratio of two modes i a n d j can be defined by eq 3
(3) As the intensities of the v,,(C-F) and v,(C-F) bands of the CFZremain strong in the FTIR LB films and the KBr spectra, we can reasonably consider that the perfluoroalkyl chain is parallel to the normal surface of the calcium fluoride plates (pv,(c-~l= 90"). From the relative intensities given by FTIR LB spectra (Iiand I V , ( c - ~=) 1)and by FTIR KBr spectra (Ai and AV,.(c-F) = 11, the pi value can be calculated according to eq 4.
For the asymmetric sulfone stretching and its symmetric counterpart, the calculated angle relative to 2 axis is estimated to k41" and f58", respectively. From the 1627cm-l band, assigned to a C-C stretching in phenyl ring, the angle value between the main axis of the phenyl ring (substituted by -SOzCaF1,) and the substrate normal is calculated to f 3 2 ". Such a result shows that the estimated angle by FTIR LB spectra (32") is near the calculated one by mechanics program PC Model 4D for isolated molecule (36") or determined by X-ray measurement (40") of substituted benzofuran in 2,6 positions and provides a strong argument that the N,N-dimethylanilino group is perpendicular to the substrate, as well as the perfluoroalkyl chain.
Conclusions We have demonstrated that it is possible to form LB films of PFSDS-023, which is a potential candidate for optical and molecular electronic applications. One can produce stable LB monolayer on the water surface; the molecules were transferred as a monolayer and multilayers onto solid supports. At the used transfer pressure, the area per molecule (42 A2/molecules)was larger than for simple perfluoroalkyl chains (28 k). As the relationship between the absorbance and the number of layers of LB films was linear up to 14layers, a constant architecture in the LB films is expected.
The UV/visible absorption and emission spectra of PFSDS-023 were blue-shifted relative to those of the isolated molecule in solution. The observed changes in absorption can be explained by a "cardpark" exciton model. The similar blue shift of the absorption spectra for the LB films transferred under various pressure (8, 25, and 35 mN/m), taken together with the fact that monolayer absorbance spectra of PFSDS-023 under diluted conditions (1:18) with CaF1dCH2)loCH20Hare similar to those obtained from the LB films of PFSDS-023, shows that H-aggregation corresponds to a minimum free energy. The blue shift of the emission spectra is attributed to a large increase of the excited state dipole moment, upon excitation, which leads to strong repulsive interactions between cofacially packed adjacent excited and groundstate dipole moments of the polar molecules. From a solvatochromic method, the dipole moment of the excited state was found to be 22.2 D, which represents approximately a 4-fold increase over the ground-state dipole moment bg= 5.2 D). Comparison of the FTIR LB spectrum with the usual IR absorbance spectrum of bulk PFSDS-023 microcrystals randomly oriented into a KBr pellet shows drastic changes: the most significant ones are the decrease ofthe phenylvibrations (C-C) stretching(l627,1594,1511,and 1484 cm-l) and vas(SOp)1371 cm-' and v,(SOd 1118cm-' bands, whereas in the C-F stretchingregion, the intensity of the v,,(CF2) and v,(CFz) bands remains strong. This behavior implies a well-defined orientation of the PFSDS0 2 3 LB molecules with respect to the substrate, with strong evidence that the transition moment of the ( C F Z ) ~ sequence of the perfluoroalkyl chain is in the plane of the substrate. The angle between the main axis of the sulfonylphenyl and the substrate normal is determined to be 32", close to the calculated one (36") with the molecular mechanics program PC Model 4D.
Acknowledgment. We thank B. Desbat of the URA 124 of CNRS, University of Bordeaux 1, for fruitful discussions in the infrared characterization part and 0. Babot (LCOO,Univ. Bordeaux I)for checkingofmultilayer and bulk IR spectra. The authors thank Professor F. De Schryver for kindly providing us with the Decan (1.0) program for decay curve analysis (T. de Roeck, N. Boens, J. Dockx, copyright 1991). LA940820N
